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Massive Fish Kill Cause Explored

OCEAN CITY- A variety of natural factors, and not likely pollution, are being blamed for a massive fish kill last weekend in the headwaters of Gray’s Creek near Bishopville, during which an estimated 50,000 menhaden died.

Maryland Department of Environment (MDE) officials this week are investigating a fish kill in Gray’s Creek that occurred last weekend, resulting in the death of around 50,000 young menhaden. According to MDE spokesman Jay Apperson, low dissolved oxygen levels in the creek from natural factors appear to be the preliminary cause of the fish kill.

“A strong northwest wind may have blown much of the water out of the creek, stranding the fish in a small amount of water,” he said. “And the concentration of fish consumed the dissolved oxygen until the level dropped below what is needed to sustain the fish.”

While the working theory related to the concentration of thousands of menhaden in a confined area dropping the dissolved oxygen levels in the creek, certain man-made factors have not been ruled out. However, Apperson said that doesn’t appear likely.

“There’s no evidence that pollutants caused the low dissolved oxygen levels,” he said.

According to MDE officials, early tests revealed the dissolved oxygen level in the creek following the fish kill was about one-third the healthy level of 5 mg per liter of water. The oxygen level in Gray’s Creek last Sunday after the fish kill was discovered was 1.5 mg per liter. Apperson said young menhaden are particularly sensitive to dramatic changes in dissolved oxygen content in the water.

“This species is very unique,” he said. “They travel in very large schools upwards of a million and they’re an open water species,” he said. “They swim through the water column and filter phyloplankton for their food. They’re adapted to swimming in open water, and when they get corralled, they don’t adapt well.”

The fish kill in Gray’s Creek last weekend was likely the result of the aforementioned scenario, according to Apperson.

“With the shee mass of them, they can get so packed in that they can’t get out,” he said. “They can also be corralled in by larger predatory fish.”

MDE officials are also exploring the time of day as a possible contributing factor to last weekend’s fish kill in Gray’s Creek.

“Another possible factor is when the sun goes down, aquatic plants stop the photosynthesis process that creates oxygen, so that lowers the dissolved oxygen levels in the water at night,” he said.